Direct Current (DC) power transmission systems are of interest to use in various situations, for instance when transferring electrical power over long distances.
In these systems there has to be provided DC breakers for security measures, for example for being able to break the current in a power line in case of a fault such as a ground fault.
Such a DC breaker will in many cases comprise one or more semiconductor devices, for instance in the form of Insulated Gate Bipolar Transistors (IGBT) being controlled using a gate control device and one or more valve control units.
Furthermore, some DC systems may be provided for covering long distances and may also be provided at high voltages such as above 500 kV, for instance in the range 500-1200 kV. This also means that the DC breaker will be placed at a high potential.
A semiconductor device may here be controlled by a control system comprising a valve control unit and a gate control device. The high potential leads to problems with galvanic isolation of the gate control device in order to be able to operate these high voltages. The isolation would normally require a large transformer, which is both bulky and expensive.
One way of addressing this is through using optical control signals. These optical signals may be provided to the gate control device, which in turn performs the control of the semiconductor devices. In this way the problem of galvanic isolation of the control signal at a high voltage is solved without having to resort to a transformer.
Optical control signals are described in a number of documents.
One example is given in U.S. Pat. No. 7,239,535, which describes a gate control device connected via optical fibres to one central valve control device. The valve control device is also described as being galvanically separated from the gate control device.
JP 62-131755 describes how firing signals from firing signal generators and other signals are combined in a light guide and supplied to a thyristor. The signals in the light guide are also supplied to a light detector of a firing signal switching circuit.
JP 59-209066 describes dual firing devices connected to gate circuits via optical fibres. Each firing circuit is connected to a corresponding gate circuit.
U.S. Pat. No. 5,629,869 describes an intelligent circuit breaker, where a breaker control unit can be redundant and use fibre optic cabling.
These documents thus describe the use of optical fibres for transmitting control signals in relation to the control of a semiconductor device.
However, it may then also be of interest to also use the optical signals for energy supply.
However, in order to be able to do this, laser sources, such as laser diodes, have to be used instead of light emitting diodes (LEDs). Because of the power required the reliability of these laser sources will then also be an issue, which needs to be addressed.
Furthermore, when gate control devices are optically powered from valve control units, the reliability becomes important. The reliability of laser sources in valve control units is typically lower compared to the reliability of LEDs or low power vertical-cavity surface-emitting lasers (VCSELs) used for control of HVDC converters.
Today the maintenance cycles are about one year long. It is therefore of interest to extend these.
The present invention is concerned with addressing the problem of handling reliability in a control system for semiconductor devices employing optical light as both power source and control signal source.